Industar-61 I-61 2.8/52 M39 mount USSR lens for rangefinder FED

Industar-61, lens for rangefinder camera
52 mm, f/2.8, M39 (LTM) mount, s/n 6967415
"FED" plant, Kharkiv, USSR

Very good condition. Lens is clear and clean, without any fungus. Front glass has cleaning microscratches. Inside there are microspecks of microdust. Body has small signs of age. Aperture ring rotates properly, focus ring rotates not uniformly and  has a backlash. Aperture blades have oil.

Camera "Zorki-4" is not included and shown as an example of mount.

This is probably the most popular lens from vast "Industar" family. I-61 is a lens with medium lens speed and multilayered anti-reflection coating. Extraheavy lanthanum crown glass "STK-6" is used. Authors of the lens - USSR academician G.Slusarev and V.Sokolova. Optical resolution of this four-lenses system is equal to six-lenses "Helios-81".
This lens was made for rangefinder "FED" and SLR "Zenit" since 1971 till 1990 at three plants: KMZ (Krasnogorsk), FED (Kharkiv), LZOS (Lytkarino).

For "FED-3,-4,-5" it was a default lens. 

"Industar-61 L/D" has 6 aperture blades; letters "L/D" mean Lanthanum/ for rangefinder. Also there was I-61 L/Z, where "Z" meant - for SLR camera.
"I-61" (without letters) has either 10 or 6 aperture blades.

Characteristics:
Lens s/number - 6967415
Number of lenses - 4 in three groups, Tessar
Angle of view - 46o 
Number of aperture blades - 10
Limit of aperturing - 1:2.8 (max), 1:16 (min)
Resolving power - (centre/edge) 45/28 lines/mm
Minimal distance of focusing - 0.42 m
Type of thread - M39x1/28.8 mount
Dimensions (mm): 58*59*59
Focal distance - 52,42 mm.
Weight - 122 g
Multilayered anti-reflection coating